Aromatic hydrocarbons, such as benzene, toluene, xylene, etc., are useful as fuels, solvents, and as feeds for various chemical processes. Of the aromatics, those having 6 to 8 carbon atoms (e.g., benzene, toluene, and xylene) are especially desired. Para-xylene (“p-xylene”) is particularly useful for manufacturing phthalic acids such as terephthalic acid, which is an intermediate in the manufacture of synthetic fibers such as polyester fibers.
One conventional process for producing aromatics involves reacting an oxygenate, such as methanol, in the presence of zeolite, such as ZSM-5, to produce water and a hydrocarbon product containing aromatics. See, e.g., C. D. Chang and A. J. Silvestri, Journal of Catalysis 47, p. 249 (1977), which discloses a process featuring high methanol conversion in approximately stoichiometric yield to produce about 44 wt. % hydrocarbon and about 56 wt. % of water, based on total product weight. Water and a portion of the hydrocarbon are condensed by exposing the product to a temperature <100° C., with the vapor being recycled to the reactor inlet. The vapor, which comprises about 20 wt. % of the hydrocarbon product, contains primarily C1-C4 paraffin and C2-C4 olefin. At a reaction temperature of about 370° C. and a liquid hourly space velocity (LHSV) of approximately 1.0 hr−1, approximately 80 wt. % of the hydrocarbon product comprises gasoline boiling-range hydrocarbons, including C6-C11 aromatics. Under these conditions, the hydrocarbon product comprises approximately 30 wt. % of C6-C8 aromatics and approximately 10 wt. % of C9+ hydrocarbon, primarily 1,2,4,5-tetramethylbenzene (i.e., durene). Durene can be removed by selectively hydrotreating the aromatic portion of the hydrocarbon product.
It is desired to convert oxygenates to hydrocarbons with an increase in the relative amount of C6-C8 aromatics in the hydrocarbon product as the C6-C8 aromatics are commercially useful. It is particularly desired to do so without increasing (i) the relative amount of durene in the aromatic portion of the hydrocarbon product to avoid extensively treating the product to remove durene toxic and/or (ii) the rate of catalyst deactivation. It is even more desired to do this while increasing the relative amount of p-xylene in the hydrocarbon product since p-xylene is the most commercially desired of the C6-C8 aromatics.